1. Field of the Invention
The present invention relates to a recording apparatus, and more particularly to a serial recording apparatus employing a stepping motor as a driving source at least for scanning movement of a recording head.
2. Related Background Art
A serial recording apparatus generally employs, for driving a carriage for moving a recording head for scanning movement for recording, a brushless motor or a stepping motor of hybrid type or permanent magnet type.
Such brushless motor generally employs a Hall element for detecting the position of magnetic poles of a rotor, for the purpose of current supply control, and an optical or magnetic encoder for detecting the rotor speed.
However, such brushless motor has been associated with the following drawbacks:
(1) The Hall element has to be aligned with the magnetic poles of the stator; and
(2) The current supply switching by the Hall element fixes the positional relationship between the Hall element and the stator, so that the current supply method to the motor becomes unflexible. For example, the position of the Hall element is electrically different by 45.degree. with respect to the magnetic poles of the stator for so-called 180.degree. current supply control and 90.degree. current supply control. Therefore, in order to effect both control methods in a same motor, the Hall elements have to be doubled in number and be positioned suitably for respective control methods.
For example the Japanese Laid-Open Patents Sho 62-193548 and Sho 62-193549 propose a stepping motor with current supply control utilizing the output of an encoder, but these patents only disclose the motor structure with an encoder in a predetermined position and do not teach any control circuit or method for driving such motor.
The U.S. Pat. No. 4,963,808 discloses a control device for a stepping motor, in which an encoder, having detection units of a number corresponding to an integral multiple of the number of the magnetic poles of the rotor, is fixed on the shaft of said rotor, then the number of said detection units of said encoder is counted in the course of rotation of said rotor at a predetermined position in the stator, and the current supply to the stator coils is switched when the obtained count reaches a predetermined value. In this manner a closed-loop control is applied on the stepping motor.
Conventional drive control for the stepping motor has been conducted by a simple open-loop control of the number of driving pulses for said motor and the frequency of said pulses.
When a stepping motor driven with such conventional open-loop control is employed as the carriage driving motor, there are generated high-pitched noises, in the course of movement of the carriage, resulting from the vibration of rotor, particularly in the hybrid type motor. Also at the starting, stopping and reversing of the carriage or of the stepping motor, there is generated a large "thud" sound because the stepping motor is started or stopped while it vibrates. These noises become a problem particularly in a quiet printer, such as an ink jet printer or a bubble jet printer.
On the other hand, the brushless motor, requiring a longer start-up time, is not suitable for the carriage driving motor which has to repeat starting, stopping and reversing in almost every line, and a high-speed recording cannot be achieved with such brushless motor.
Also the U.S. patent application Ser. No. 302,196 filed Jan. 27, 1989, now U.S. Pat. No. 4,928,050 proposes a recording apparatus utilizing a stepping motor as the drive source for moving the recording head for scanning, provided with detection means for detecting the angular position of the rotor of said stepping motor, and control means for closed-loop control of drive of said stepping motor according to the result of detection by said detection means.
In said closed-loop control of the stepping motor, an encoder is mounted on the shaft of the stepping motor, and the output signals of said encoder are counted for detecting the rotary position and the motor driving signal is switched at a predetermined count.
Said encoder generates, in the course of rotation of the rotor, pulse signals of which number is proportional to the amount of rotation. An example of such pulse signals are shown in FIG. 7.
In FIG. 7, the signals from the encoder are composed of two-phase signals A, B, which are converted, by a decoding circuit, into a rotational direction signal and a rotational amount pulse signal shown also in FIG. 7. Receiving these signals, the control unit of the printer determines the rotating speed of the carriage motor by measuring the interval of the rotation amount signal with an internal timer. The control unit detects the rotation state of the carriage motor, constantly compares the actual rotating speed of said motor with a speed for speed control stored in advance in a memory, and generates a motor control signal for example by a PWM signal. For effecting such control, the control unit generally effects PI (proportional integration) control in which released is the sum of an amount proportional to the error between the control speed (designated speed) and the actual speed, and an amount proportional to the integrated value of said error.
However, in a serial printer, the waiting time of the carriage motor from the stopping to the next starting, for example at the reversing of the carriage, becomes shorter as the recording speed becomes higher. In certain recording data, the carriage may be started in the opposite direction almost at the same time with the stopping. The speed change in such case is shown in FIG. 15.
FIG. 15 shows the speed change of the carriage in case of starting a recording process at a constant speed in the reversed direction, after a recording process at a high speed.
In FIG. 15, a line 61 indicates the speed of the carriage motor instructed by a speed control unit, while a line 62 indicates the actual speed. When the carriage moving at a high speed enters a deceleration stage, the carriage tends to maintain the current speed, by the inertia of the motor and the carriage. For this reason, the decrease of the actual speed (62) is delayed from the instructed speed (61). Thus, as shown at a time 63, even after the instructed speed reaches, zero, the carriage still has a certain retentive speed as indicated by "a".
Then, even after a next acceleration is instructed by the control unit, the carriage still continues deceleration by inertia as indicated by the lines 62, so that the discrepancy in speed from the instructed speed, indicated by the line 61, becomes even larger. Consequently the integrated error also increases progressively. As a result, the control unit increases the instructed speed in order to reduce the error in speed, whereby the discrepancy between the actual motor speed and the instructed speed becomes even greater. Consequently the carriage motor is controlled with a large acceleration, thus eventually resulting in generation of large noises.
In such control, when the carriage speed becomes constant, the integrated speed error accumulated to this point is discharged at once to result in an overshooting and ensuing speed variations. The recording operation, during the presence of such speed variations, may result in fluctuation of the recording position, eventually deteriorating the quality of recording.
The recording operation may be started not during but after said overshooting phenomenon, but such method sacrifices the advantage of short start-up of the carriage movement in the acceleration region.